Semi-conductor integrated circuit devices typically comprise a number of components, including: (a) a silicon wafer that includes at least one semi-conductor element (e.g., a transistor) and, on one of its surfaces, a thin layer of a non-conductor, typically silicon dioxide; (b) a number of interconnection conductor layers that are made from a conductor metal, such as aluminum, tungsten or titanium; and (c) a number of passivating layers that are made from a non-conducting material, such as silicon dioxide.
The microelectronic circuit of such semi-conductor devices comprises the semi-conductor element of the silicon wafer and the interconnection conductor layers. During manufacture, those interconnection layers are built up, layer by layer, on the silicon wafer, with each layer having a pattern prescribed by the circuit design.
Also during manufacture, passivating layers are provided between adjacent interconnection conductor layers. The interconnection conductor layers are connected to each other and to the semi-conductor element through holes in the passivating layers and the thin layer of non-conductor on the surface of the silicon wafer. Such holes are provided in the passivating layers and the thin layer of non-conductor on the surface of the silicon wafer in a predetermined pattern prescribed by the circuit design.
The fabrication of a semi-conductor device requires a method for accurately forming the patterned layers that comprise the device. The photoengraving method by which this is accomplished is known as photolithography.
Materials known as "photoresists" are used in photolithography. Photoresists that are conventionally used in connection with the manufacture of semi-conductor devices are materials whose solubility characteristics in certain solvents, which are called "developers", are affected by exposure to ultraviolet radiation. A "negative photoresist" is a material that prior to exposure to ultraviolet radiation is soluble in developer, but after exposure is insoluble in developer. In contrast, before exposure a "positive photoresist" is insoluble in developer, but after exposure to ultraviolet light it becomes soluble in developer.
Photoresists are used in connection with the forming of the pattern of each of the various layers in a semi-conductor device. For example, a negative photoresist may be used to pattern the silicon dioxide layer of an oxidized silicon wafer by the process described below.
Firt, the negative photoresist is applied to the oxidized surface of the silicon wafer by: (a) dissolving the photoresist in a suitable solvent; (b) applying a drop or several drops of the resulting photoresist solution onto the oxidized surface of the wafer; (c) rapidly spinning the wafer to spread a thin film of the solution across the oxidized surface of the wafer; and (d) evaporating the solvent from the solution to leave a thin film of the negative photoresist on the oxidized surface of the silicon wafer. Typically, the photoresist is then heat treated to dry it out thoroughly and to improve its adhesion to the silicon wafer.
The negative photoresist layer is next selectively exposed to ultraviolet radiation. This may be accomplished by positioning a patterned mask into juxtaposition with the negative photoresist layer and then flooding the mask with ultraviolet light. As previously mentioned, the solubility characteristics of the negative photoresist are altered by the exposure, i.e., after exposure, the exposed portion of the photoresist is insoluble in a developer solution while the non-exposed portion remains soluble in the developer solution.
After exposure, the negative photoresist is developed (i.e., it is washed in a developer solution) to remove the portion of the photoresist layer that was not exposed to ultraviolet radiation. The photoresist pattern that remains after development may then be hardened further by heat treatment.
The wafer, with its photoresist pattern on it, is then placed in a solution (e.g., a hydrofluoric acid solution) that dissolves or etches the silicon dioxide layer wherever it is not protected by the photoresist, but does not attack to any significant extent the photoresist itself, the portion of the silicon dioxide layer under the photoresist, or the portion of the silicon wafer under its silicon dioxide layer. The wafer is then rinsed and dried and the remaining photoresist pattern is removed by further chemical treatment, leaving a silicon wafer with a silicon dioxide layer in a prescribed pattern on one of its surfaces.
Other layers of the semi-conductor device may be patterned by procedures similar to that described above. For example, aluminum interconnection conductor layers may be patterned by using a warm phosphoric acid solution to dissolve or etch the portion of the aluminum layer that is not protected by the photoresist after selective exposure and development of the photoresist.
Substantial efforts have been devoted over the years to reducing semi-conductor integrated circuit devices to the smallest possible size. These efforts have been successful to a large extent due to a number of advances in the art including, inter alia, the miniaturization of circuit elements and their interconnections.
One consequence of the successful efforts to reduce the size of semi-conductor devices has been an ever increasing need to pattern each layer of the multi-layered structure precisely in accordance with the specifications for that layer and to position each pattern accurately with respect to the patterns in the other layers of the semi-conductor device. Any substantial deviation from the specifications for the semi-conductor device in these regards can result in a device that does not function in the proper way.
An operation in the manufacture of semi-conductor devices that must be precisely controlled to achieve a defect free device is the selective exposure of the various photoresist layers utilized in the construction of the device. In particular, it is important that the photoresist be exposed in those areas dictated by the specifications for the device, and only in those areas.
A phenomenon that has inhibited the precise exposure of photoresist layers to ultraviolet light is the internal reflections that occur in a semi-conductor device as a consequence of the exposure. Such reflections occur when ultraviolet light that has passed through a photoresist layer is reflected back from a reflective surface in the semi-conductor device. Such a reflective surface may be a silicon dioxide layer on the silicon wafer itself, a metal (e.g., aluminum) interconnection conductor layer, or a reflective passivating layer (e.g., a silicon dioxide passivating layer).
Typically, ultraviolet light is scattered when it is reflected as described in the preceding paragraph. As a consequence, portions of the photoresist are exposed by the reflected light that were not exposed when the ultraviolet light initially passed through the photoresist. This, of course, may result in a photoresist pattern after development that does not correspond to the pattern prescribed by the mask through which the photoresist was exposed.
A number of techniques have been proposed to eliminate or minimize reflections of the type described above. For example, one technique that has been proposed has been to apply a thin coating of a polyimide, which includes a dye, on a silicon wafer before a positive photoresist is applied to the wafer (see Brewer et al., "The Reduction of the Standing-Wave Effect in Positive Photoresists", Journal of Applied Photographic Engineering, Vol. 7, No. 6, at pp. 184-86 (December 1981)). In this technique, the dye is said to absorb the light that passes through the photoresist during exposure and the polyimide is said to provide a smooth coating that can be etched with a standard photoresist developer without extra processing steps.